My neighbours’ baby has been constantly making noises for 3 hours now. Does it have to be quiet?

Answer 1

Talk to your neighbors about that and maybe ask them to be more considerate of others.

Answer 2

Does it have to? No. It's a baby. It doesn't understand what "be quiet" means. It doesn't understand that making noises bothers you. 

Answer 3

No.  Buy some ear plugs.